System for reducing background developer deposition in an electrostatic copier

ABSTRACT

In an electrostatic copier, a direct-current-voltage source and associated circuitry for applying to a biasing electrode in a liquid developer tank and to a ground-insulated conductive mirror-return sector of the surface of a rotating photoconductive drum the same potential as exists on background, non-imaged, regions of the charged and exposed photoconductive surface to reduce background deposition of developer. Means are provided for measuring this background potential and for adjusting the voltage on the biasing electrode and mirror-return sector of the drum to match this measured value.

United States Patent Smith 1 Jan. 1, 1974 1 SYSTEM FOR REDUCING BACKGROUND 3,576,623 4/1971 Snelling 117/37 LE x DEVELOPER DEPOSITON IN AN 3,411,482 11/1968 Brodie 117/37 LE X ELECTROSTATIC COPIER Inventor: Ian E. Smith, Lockleys, Australia Savin Business Machines Corporation, Valhalla, N.Y.

Filed: Nov. 17, 1972 Appl. N0.: 307,572

Assignee:

US. Cl. 355/10, 355/3, 96/1 LY, 117/37 LE, 118/D1G. 23

Int. Cl G03g 15/10 Field of Search 355/10, 3; 96/1 LY; 117/37 LE; 118/637, DIG. 23

References Cited UNITED STATES PATENTS Sato et a1 117/37 LE Matkan et a1 117/37 LE X Primary ExaminerRobert P. Greiner Att0rneyHenry L. Shenier et a1.

[5 7] ABSTRACT In an electrostatic copier, a direct-current-voltage source and associated circuitry for applying to a biasing electrode in a liquid developer tank and to a ground-insulated conductive mirror-return sector of the surface of a rotating photoconductive drum the same potential as exists on background, non-imaged, regions of the charged and exposed photoconductive surface to reduce background deposition of developer. Means are provided for measuring this background potential and for adjusting the voltage on the biasing electrode and mirror-return sector of the drum to match this. measured value.

10 Claims, 3 Drawing Figures SYSTEM FOR REDUCING BACKGROUND DEVELOPER DEPOSITION IN AN ELECTROSTATIC COPIER BACKGROUND OF THE INVENTION In the operation of one form of electrostatic copying machine an insulated organic photoconductive surface is first uniformly charged by the ionic discharge of a corona charger to a potential of approximately 800 volts, and is then exposed to a pattern of light derived from an original to be copied. The photoconductive surface becomes conductive in those regions upon which light impinges and the electrostatic charge thereon leaks away in those regions. In this manner, following the exposure step, a latent electrostatic image is formed upon the organic photoconductive surface, reproducing in non-visible, charged regions the corresponding opaque images of the original. The organic photoconductive surface is then developed by a suitabletoner to produce a visible and transferable replica of the original. Toner particles in a liquid carrier adhere to the charged regions of the photoconductive surface. Unfortunately, owing to the discharge properties of an organic photoconductive surface, the non-imaged areas retain some charge following their exposure to light. A residual static charge of approximately 100 volts potential remains in these background areas, causing toner particles which are applied to the photoconductive surface to adhere thereto. Thus, on development, the background areas of the copy appear grayish in color and authentically reproduced contrast is unattainable.

As indicated in the copending patent application of Smith et al., for Self-Cleaning Developer Applicator, Serial No. 213,885 filed Dec. 30, 1971, tests have shown that an initial discharge from approximately 800 volts to approximately 100 volts requires an exposure of about 3 footcandle-seconds. However, to discharge an organic photoconductor to about volts requires an exposure of 12 footcandle-seconds. Complete discharge of an organic photoconductor requires an exposure of the material of inordinate duration. To achieve such a discharge the exposure time would have to be so long, or the optical system so large, or both, as to make the system of electrostatic copying employing an organic photoconductor entirely impracticable.

Recognizing the problem of employing organic photoconductors in an electrostatic copying system, methods have been developed, in the prior art, in which an electrical field is applied to counteract the effect of background potential on the organic photoconductor, while at the same time adequately developing the desired image areas. In setting up such a counter-field, a bias potential is applied between a conductive substrate carrying the organic photoconductor and a conductive applicator electrode. While application of the counterfield in the manner described above had the initially successful effect of overcoming the background potential in the photoconductor, it resulted in the accumulation of toner particles on the electrode, and consequently the system became ineffective and the quality of copies produced rapidly deteriorated. The copending patent application of Smith et al. shows an arrangement for continuously cleaning the biasing electrode to remove toner particles which otherwise would collect thereon under the influence of the biasing potential.

Consideraton has also been given in the prior art to a system for neutralizing background potentialwithout fouling a biasing electrode in which a counter-field voltage is again applied between a toner tank electrode and the conductive substrate backing a photoconductive surface. However, this system works only when the residual voltage present in the background regions of the latent electrostatic image exists over 360 of the photoconductive surface of the drum. Unfortunately, this condition rarely exists inasmuch as up to of the drums surface is a mirror-return sector which receives no corona charge. Under normal operating conditions the mirror-return sector is at ground potential, while the photoconductive surface carries a residual voltage in its non-imaged, background areas. The rotation of the mirror-return sector past the biasing electrode in the toner tank, will cause toner particles to be deposted thereon. The counter-field potential to which a biasing electrode is raised may be switched off during this interval, but must again be switched on as the leading edge of the latent electrostatic image on the photoconductive surface of the drum enters the developing station. Since the mirror-return sector may extend over as much as 90 of the drums arcuate surface the leading edge of the latent electrostatic image will always enter the developing station while a portion of the mirrorreturn sector is still moving through the developing station. Thus, switching on the counter-field potential at this point will cause toner particles to be deposited on the biasing electrode.

I have now developed a method and apparatus for neutralizing the effect of residual potential in background regions of an organic photoconductor, using a counter-field producing electrode, without the need for an electrode cleaning apparatus. l have achieved the object of authentically reproducing the contrast of the original, without the need of a complicated and relatively inaccessible electrode cleaning apparatus.

SUMMARY OF THE INVENTION One object of my invention is to provide a developer applicator for use with an organic photoconductive imaging surface.

Another object of my invention is to provide a developer applicator which substantially eliminates theeffect of background potential in an image carrying organic photoconductive surface.

A further object of my invention is to provide a developer applicator without the need of a cleaning apparatus to inhibit the buildup of toner particles on the applicator biasing electrode.

Still another object of my invention is to provide a developer applicator for use with an organic photoconductive imaging surface which produces clear copies of authentic contrast over a relatively long period of operational use.

Other and further objects .of my invention will appear from the following description.

In general, my invention provides a direct current voltage source and associated circuitry which applies to a biasing electrode in a liquid developer tank and to the mirror-return sector on a portion of the surface of rotating copy machine drum, a potential which neutralizes the residual field in background regions of the charged and exposed photoconductive surface. This arrangement overcomes the undesirable effect of the residual charge on non-image areas of the photocon' ductive surface. My invention eliminates the need for continuously cleaning the biasing electrode to remove toner particles which would otherwise collect thereon, under the influence of the biasing potential alone.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings which form part of the instant specification and which are to be read in con junction therewith and in which like reference numerals are used to indicate like parts in the various views:

FIG. 1 is a schematic partial view of one form of electrostatic copying system incorporating my developer applicator.

FIG. 2 is a diagrammatic view illustrating the discharge characteristic of a typical organic photoconductor.

FIG. 3 is a fragmentary perspective view of the drum and developing station of a photocopy machine incorporating my invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIGS. 1 and 2 of the drawings, one type of electrostatic copying machine, indicated generally by the reference character 10, which may incorporate my developer applicator, includes a drum 12 carried by a shaft 14 for rotary movement in the direction of the arrow A. The drum 12 carries a film 16 of a suitable photoconductive material which may, for example, be an organic photoconductor, and also carries a non-photosensitive mirror-return section over as much as 180 of its arcuate surface. The drum s interior 13 is made of a conductive material over which 1 form my photoconductive layer. I insulate the mirror-return sector 15 from substrate 17 and from film 16 by insulation 17. Additionally, I provide circular strips of insulating material to form two insulative rims 19 along the edges of the drum 12 extending over the drums circumference between the edges of the mirror-return sector 15.

In the operation of the machine 10, as the drum 12 rotates, the surface 16 moves past an exhaust exposure source 18 of light which is adapted to remove any charge remaining thereon as a result of a previous operation. The drum 12 moves the surface 16 past a corona discharge system 20 which, as is known in the art, is adapted to apply a predetermined electrostatic charge to the photoconductive film 16. After receiving the charge, the film passes an exposure system 22 which exposes the film to a light pattern derived from the original to be copied, so that surface charge is lost in non-image areas while being retained in areas to be developed.

The mirror-return section 15 of the surface of the drum 12 is a conductive but non-photosensitive region which is arranged to rotate past the optical exposure system 22 during the time in which the scanning mirror of the optical exposure system returns to its initial position in preparation for a new operation. The mirrorreturn section is normally used to automatically switch on the corona charger 20. The mirror-return sector may also be of such length as to entirely rotate past the corona charger 20 during the time required for the corona current to rise to its operating threshold. Thus the mirror-return sector receives no charge from the corona.

AFter leaving the exposure system 22 the surface 16 carrying the latent electrostatic image moves through the developer system indicated generally by the reference numeral 24 which applies a developer to the surface of the film 16 in order to develop the image. The system 24 may, for example, be of the type disclosed in the copending application of Smith et al, for "Method of Contact Transfer of Developed Electrostatic Images and Means for Practicing Same, Ser. No. l55,l08 filed June 21, I971, in which the developer applied by the system 24 is made up of a tacky toner of a suitable carrier. The developer unit includes a tank 28 which holds a supply of liquid developer for application to the latent electrostatic image, and an overflow trough 26 which collects excess liquid developer removed from the photoconductive surface.

Paper or other material from a roll 30 moves through a heating unit 32 around a guide roll 34 and is pressed into contact with the surface of the film 16 by means of pressure rollers 36 and 38. As the tacky toner liquid leaves the developer station it is pressed into contact with the paper in engagement with the surface of the film 16. Owing to the fact that the tacky toner has greater adhesion affinity for the surface of the paper than for the photoconductive film, the image is transferred to the paper. Following the reception of the image, the paper moves around guide rolls 40 and 42 which deliver the copy. As is more fully pointed out in the Smith, et al., (II) application, a cutter may be employed to cut the copy to a desired length. In use, the machine 10 produces one copy per revolution of the drum 12.

I employ an apparatus of the type disclosed in the copending application of Smith et al., (III) for Apparatus for Developing Electrostatic Images," Ser. No. 212,155 filed Dec. 27, I971, or an apparatus of the type disclosed in the copending application of Smith et al. for a Centrifugal Impingement Mill," Ser. No. 295,115 filed Oct. 5, 1972, for supplying an emulsion of tacky toner particles in a carrier to the developer tank 28 of the developer applicator apparatus 24, and which is connected thereto through pipes 48 and 50. The toner supply and circulation system has been omitted from FIG. 1 because the construction and operation of the liquid supply and circulation system do not per se form part of my invention.

As I have indicated hereinabove, the machine 10 with which my developer applicator is used, employs an organic photoconductor 16 on which the latent image is formed. Referring now to FIG. 2, l have shown an idealized discharge curve of a typical organic photoconductor. In this figure, the initial voltage to which the film is charged is represented by the ordinate while the logarithm of the light exposure in footcandle-seconds is represented by the abscissa. As can be seen from the FIGURE, with the film initially charged to about 800 volts, 3 foocandle-seconds of exposure is required to discharge the film to about volts. To discharge the film further to about 10 volts requires 12 footcandleseconds of exposure. It can further be seen from the characteristic curve that an exposure of extremely long duration is required to completely discharge the film. I apply a potential to the conductive mirror-return sector 15 on the drum l2 and to an electrode 29 positioned in the developer applicator tank 28, in a manner to be described hereinbelow, which effectively raises the abscissa of the plot of FIG. 2, counteracting the ef fect of the residual charge remaining on non-image areas of the photoconductive surface after an exposure. For example, I may apply such a bias as to raise the abscissa to the level indicated by the broken line in FIG. 2, and thus, to require an exposure of only 3 footcandie-seconds to develop the latent electrostatic image with authentically reproduced contrast.

My invention employs circuitry which maintains the mirror-return sector at the same potential to which the biasing electrode is raised, so that no toner particles are deposited on the biasing electrode during the passage of the mirror-return sector through the developing station. In this manner, the voltage to which the biasing electrode is raised need not be switched off as the mirror-return sector enters the developing station, and need not be switched back on, with resultant electrode fouling, as the leading edge of the latent electrostatic image enters the developing station.

The magnitude of the biasing voltage is a function of the configuration of the electrode 29, the spacing between the electrode 29 and the magnitude of the residual field. For a particular embodiment it is readily determined experimentally by adjusting the bias voltage until copy with a cleanbackground is produced. In a specific instance, for the electrode configuration shown, with an electrode-to-drum surface spacing of 0.03 inch and a residual background charge potential of about 50 to 70 volts a bias voltage of 100 volts produced copy having substantially no background deposition. The surface of the drum was charged negatively from a 5.5 Kv corona source and copies were run at a speed of 45 feet per minute. Where the corona charge was negative the bias voltage was negative.

I connect the electrode 29 and spaced electrical contacts 31a, 31b and 31c to one terminal of a direct current voltage source 56. The electrical respective contacts or brushes 31a and 31b are located adjacent the leading and trailing edges of the overflow trough 26 and are disposed beneath the rim of the rotating drum 12, making contact therewith, as shown in FIG. 3. Contact 310, which also contacts the rim of the drum, is located adjacent to the leadingedge of the corona charger 20. Contact 31c ensures that the bias potential is applied to the sector during its passage under the charger to prevent charge buildup on the sector if isolated electrically. In this manner, I assure that the mirror-return sector is continually in communication with the negative terminal of the voltage source 56 as it rotates through the developing zone. The photoconductive surface 16 of the drum 12 does not receive any additional potential from the source 56 because it is insulated therefrom by the circular insulating strip 19. The other terminal of the voltage source 56 is connected to the conductive substrate, backing the organic photoconductor, by way of a contact 33.

As the mirror-return section 15 rotates past the developing station it is connected in parallel with the toner tank electrode 29 to the negative terminal of the voltage source 56. The mirror-return sector and toner tank electrode are thereby raised to the same potential for the duration of the mirror-return sectors transit through the developing station. I adjust the magnitude of this voltage to neutralize the residual field present in the non-imaged areas .of the photoconductive surface. In this manner, no electric field will exist between the mirror-return sector 15 and the toner tank electrode 29 as the mirror-return sector passses through the developing station, and consequently toner particles will not be attracted to the toner tank electrode, or to the mirror-return sector.

' I arrange the drum 12 so that the total area of the photoconductive surface 16 extending between the limiting edges of the mirror-return sector 15 is charged and then exposed to the light pattern derived from the original. Following the exposure step a residual potential will remain on background areas of the photoconductive surface as explained hereinabove. I may set the toner tank electrode 29 to a predetermined counterfield potential or alternatively, I may vary the counterfield potential according to the actual magnitude of the residual potential present on the background areas of the photoconductive surface. The residual field neutralization may be controlled automatically by means of a surface-charge field sensor 52 positioned above a non-imaged area of the photo-conductive surface adjacent to the insulating rim 19 of the drum 12, which receives corona charge and is subsequently discharged by light exposure. I connect the surface-charge sensor 52 in series to a high impedance, direct current amplifier 54 which is subject to zero drift and insulated from stray electric fields. I arrange the output of the amplifier 54 to control the counter-field produced by the source 15 to match that measured by a surface-charge sensor 52, and to apply this potential to the electrode 29 and mirror-return sector 15. Alternatively, I may incorporate within the amplifier 54 circuitry which measures the difference between the counter-field generated by the source 56 and the residual field measured by the surface-charge sensor 52, and which adjusts the potential at the electrode 29 and mirror-return sector 15 to achieve residual field neutralization.

I do not maintain the mirror-return sector at the same potential as that of the toner tankelectrode throughout the entire unit rotation of the drum. The potential applied to the mirror-return sector must be identical to that of the toner tank electrode only during the time in which the mirror-return sector passes through the developing station. I ground the mirror-return sector during the time in which it rotates past the corona charger so that corona current does not alter the applied d.c. voltage. The change in voltage applied to the mirrorreturn sector may be easily achieved by appropriately arranged switches. However, if a low impedance voltage source is employed, the switching procedure becomes unnecessary, as the corona current magnitude of approximately 10 milliamps is too small to affect the d.c. voltage source.

It will be seen that l have accomplished the objects of my invention. I have provided means for inhibiting the deposition of toner particles on non-imaged background regions of a photoconductive surface by neutralizing the residual field remaining thereon following the exposure of the surface to light. My invention accomplishes this result by maintaining a biasing electrode and a mirror-return sector on the surface of a photoconductive drum at a potential to produce a counter field which neutralizes the residual field in the non-imaged areas of the photoconductor. Consequently, no net electric field exists between the biasing electrode and the mirror-return sector or the nonimaged areas of the photoconductive surface as they rotate past the biasing electrode, and as a result, toner particles do not adhere thereto. This arrangement eliminates the need for continually cleaning the biasing electrode.

It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of my claims. It is further obvious that various changes may be made in details within the scope of my claims without departing from the spirit of my invention. It is, therefore, to be understood that my invention is not to be limited to the specific details shown and described.

Having thus described my invention, what I claim is:

1. Apparatus for inhibiting the development of background areas of residual electrical potential in a latent electrostatic image including in combination, a member having a photoconductive first surface portion adapted to receive said latent electrostatic image and a conductive second surface portion, a developer unit adapted to apply developer to a surface, means for moving said surface portions relative to and adjacently past said developer unit, a biasing electrode associated with said developer unit, means for applying a potential to said biasing electrode and means for applying said potential to said second surface portion in the course of its movement adjacent to said developer unit.

2. An apparatus as in claim 1 in which said member is a rotating drum the surface of which is formed with said photoconductive first surface portion and said conductive second surface portion, and insulating means for insulating said photoconductive first surface portion from said conductive second surface portion.

3. Apparatus as in claim 1 in which said developer unit comprises means for holding a supply of developer, means for applying said developer from said supply to said latent image and means mounting said biasing electrode in said unit.

4. Apparatus as in claim 2 including a pair of electrical contacts connected in parallel with said biasing electrode and disposed adjacent the leading and trailing edges of said developer unit to contact said conductive second surface portion of said drum.

5. Apparatus as in claim 4 including a surfacecharge-sensing detector disposed adjacent said rotating drum to measure the residual background potential of a charged and exposed area of said photoconductive surface portion and voltage control means responsive to output of said detector for controlling the voltage applied by said voltage source to said biasing electrode and said conductive surface portion as its periphery engages one of said electrical contacts during its transit through the developing unit.

6. Apparatus as in claim 5 in which said surfacecharge-detector measures the absolute value of said background potential and in which said voltage control means adjusts the voltage applied to said electrode and to said conductive second surface portion to the value of said measured potential.

7. Apparatus as in claim 1 including a corona charging unit positioned adjacent to said member, and a first electrical contact connected in parallel with said biasing electrode and positioned adjacent to the leading edge of said charging unit to contact said second surface portion as it moves thereby.

8. Apparatus as in claim 7 including second and third electrical contacts connected in parallel with said biasing electrode and positioned respectively adjacent to the leading and trailing edges of said developer unit to contact said second surface portion as it moves thereby.

ll. Electrostatic copying apparatus including in combination, a member having a photoconductive surface portion and a conductive surface portion, a charger adapted to apply a charge to a surface moving adjacent thereby, an optical exposure unit adapted to project an image onto said charged surface to produce a latent electrostatic image thereon, a developer unit adapted to apply developer to said surface to develop the latent electrostatic image, an electrode associated with said developer unit, means for applying a potential to said electrode, means for driving said member successively to move said surface portions past said charger and said exposure unit and said developer unit, and means for applying said potential to said conductive surface portion in the course of its movement past said developer unit.

10. Apparatus as in claim 9 in which an organic photoconductor provides said photoconductive surface portion, said organic photoconductor retaining a residual background potential after exposure, and in which said potential applied to said electrode substantially 

1. Apparatus for inhibiting the development of background areas of residual electrical potential in a latent electrostatic image including in combination, a member having a photoconductive first surface portion adapted to receive said latent electrostatic image and a conductive second surface portion, a developer unit adapted to apply developer to a surface, means for moving said surface portions relative to and adjacently past said developer unit, a biasing electrode associated with said developer unit, means for applying a potential to said biasing electrode and means for applying said potential to said second surface portion in the course of its movement adjacent to said developer unit.
 2. An apparatus as in claim 1 in which said member is a rotating drum the surface of which is formed with said photoconductive first surface portion and said conductive second surface portion, and insulating means for insulating said photoconductive first surface portion from said conductive second surface portion.
 3. Apparatus as in claim 1 in which said developer unit comprises means for holding a supply of developer, means for applying said developer from said supply to said latent image and means mounting said biasing electrode in said unit.
 4. Apparatus as in claim 2 including a pair of electrical contacts connected in parallel with said biasing electrode and disposed adjacent the leadiNg and trailing edges of said developer unit to contact said conductive second surface portion of said drum.
 5. Apparatus as in claim 4 including a surface-charge-sensing detector disposed adjacent said rotating drum to measure the residual background potential of a charged and exposed area of said photoconductive surface portion and voltage control means responsive to output of said detector for controlling the voltage applied by said voltage source to said biasing electrode and said conductive surface portion as its periphery engages one of said electrical contacts during its transit through the developing unit.
 6. Apparatus as in claim 5 in which said surface-charge-detector measures the absolute value of said background potential and in which said voltage control means adjusts the voltage applied to said electrode and to said conductive second surface portion to the value of said measured potential.
 7. Apparatus as in claim 1 including a corona charging unit positioned adjacent to said member, and a first electrical contact connected in parallel with said biasing electrode and positioned adjacent to the leading edge of said charging unit to contact said second surface portion as it moves thereby.
 8. Apparatus as in claim 7 including second and third electrical contacts connected in parallel with said biasing electrode and positioned respectively adjacent to the leading and trailing edges of said developer unit to contact said second surface portion as it moves thereby.
 9. Electrostatic copying apparatus including in combination, a member having a photoconductive surface portion and a conductive surface portion, a charger adapted to apply a charge to a surface moving adjacent thereby, an optical exposure unit adapted to project an image onto said charged surface to produce a latent electrostatic image thereon, a developer unit adapted to apply developer to said surface to develop the latent electrostatic image, an electrode associated with said developer unit, means for applying a potential to said electrode, means for driving said member successively to move said surface portions past said charger and said exposure unit and said developer unit, and means for applying said potential to said conductive surface portion in the course of its movement past said developer unit.
 10. Apparatus as in claim 9 in which an organic photoconductor provides said photoconductive surface portion, said organic photoconductor retaining a residual background potential after exposure, and in which said potential applied to said electrode substantially equals said background potential. 